1. Field
The invention relates to a process for converting synthesis gas to liquid hydrocarbon mixtures useful as distillate fuel and/or lube base oil by contacting the gas with multiple catalysts in a mixed bed arrangement within a single reactor.
2. Description of Related Art
The majority of combustible liquid fuel used in the world today is derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply.
Alternative sources for developing combustible liquid fuel are desirable. An abundant resource is natural gas. The conversion of natural gas to combustible liquid fuel typically involves a first step of converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. Fischer-Tropsch synthesis is a known means for converting syngas to higher molecular weight hydrocarbon products. Fischer-Tropsch diesel has a very high cetane number and is effective in blends with conventional diesel to reduce NOx and particulates from diesel engines, allowing them to meet stricter emission standards.
Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C21+ wax, also referred to herein as “Fischer-Tropsch wax” or simply “wax,” which must be hydroprocessed to provide distillate fuels. Often, the wax is hydrocracked to reduce the chain length, and then hydrotreated to reduce oxygenates and olefins to paraffins. Hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, for example, the distillate fuel range, hydrocracking of these hydrocarbons is undesirable.
Considerably different process conditions are required for hydrocracking and hydroisomerization of Fischer-Tropsch wax using relatively acidic catalysts such as ZSM-5 than for Fischer-Tropsch synthesis. For this reason commercial Fischer-Tropsch plants using fixed bed reactors require separate reactors for the Fischer-Tropsch synthesis and for the subsequent hydrocracking of the resulting wax, and complicated and expensive separation schemes may be required to separate solid wax from lighter products.
It would be advantageous to provide a process in which a desired liquid hydrocarbon product in the distillate fuel and/or lube base oil range, having a minimum of lighter product and no solid phase wax, is synthesized within a single reactor bed at a common set of conditions.